1. Field of the Invention
The present invention relates to metal seal flange assemblies, and more particularly, to metal seal flange assemblies for connecting, for example, ports of a plasma vacuum vessel of a nuclear fusion apparatus and pipes which extend from various other apparatuses (e.g., vacuum pump, measuring instruments, etc.).
2. Description of the Related Art
In a nuclear fusion apparatus, nuclear fusion is caused in a state such that the inside of a plasma vacuum vessel is kept at ultra high vacuum and at high temperature. The vacuum vessel is provided with a number of ports of various types These ports are connected with pipes which extend from various apparatuses (e.g., vacuum pump, measuring instruments, etc.). In some cases, a blank cover and a transparent plate for the observation of the interior of the vessel may be attached to any of the ports
In connecting the ports and the pipes from the various apparatuses, the inside of the ports must be sealed and isolated from the outside so that it is kept in an ultrahigh vacuum free from impurities. Satisfactory sealing performance must be maintained even though the inside of the ports is at high temperature. Accordingly, metal seal flange assemblies each having a metal gasket are used to connect the ports and the various apparatuses.
Each flange assembly has a pair of flanges fixed to a port and an end of a pipe. A pair of ring-shaped knife edges are formed individually on the respective opposite surfaces of the two flanges. A ring-shaped copper gasket is interposed between the two knife edges. When a plurality of bolts, penetrating the flanges and arranged in a ring, are tightened, the flanges are fixed to each other. Thereupon, the knife edges bite into the copper gasket, and a linear sealing load from the extremities of the ring-shaped knife edges bears on the ring-shaped gasket. The linear sealing load, which acts on the whole circumference of the gasket, is defined by a value obtained by dividing the load from the knife edges acting on the copper gasket by the circumference of the extremity of each knife edge. Thus, the inside of the ports are sealed and isolated from the outside so that it is kept in an ultrahigh vacuum free from impurities.
The sealing performance depends on the size and distribution of the linear sealing load In order to obtain satisfactory sealing performance, therefore, the sealing load must have a predetermined value, and the distribution of the sealing load must be uniform with respect to the circumferential direction. The size and distribution of the linear sealing load depend on the rigidity of the flanges (i.e., thickness and material of the flanges), the number of bolts, the bolt tightening torque, and the material of the gasket.
Conventionally, there are recommendation standards for the flange thickness, bolt number, etc. (JVIS-003 entitled "Shape & Dimensions of Bakable Flanges for Vacuum Apparatuses," established on Dec. 1, 1982, by the Japan Vacuum Association), which are used to maintain satisfactory sealing performance. Table 1 shows these standards.
TABLE 1 __________________________________________________________________________ Bolt Pitch Knife Edge Linear Linear Outside Flange Bolt Circle Circle Sealing Sealing Nominal Diameter of Thickness Diameter Diameter Diameter Load W.sub.S1 Load W.sub.S2 Diameter Flange (mm) t (mm) by Number B (mm) De (mm) (N/mm) (N/mm) __________________________________________________________________________ 40 70 13 M6 .times. 6 58.7 42.0 146 204 63 114 18 M8 .times. 8 92.1 77.2 224 267 100 152 21 M8 .times. 16 130.2 115.3 318 359 160 203 22 M8 .times. 20 181.0 166.1 286 312 200 253 25 M8 .times. 24 231.8 216.9 268 286 __________________________________________________________________________
In Table 1, linear sealing load W.sub.S1 is given by EQU W.sub.S1 =.sigma..multidot.n.multidot.S/.pi.B (N/mm). (1)
Also, linear sealing load W.sub.S2, which is not provided by JVIS-003, is given by EQU W.sub.S2 =.sigma..multidot.n.multidot.S/.pi..multidot.De (N/mm). (2)
Here .sigma. is bolt tensile stress (MPa); n, bolt number; S, effective cross-sectional area (mm.sup.2) of bolts; B, bolt pitch circle diameter (mm); and De, knife edge circle diameter (mm).
The values of the linear sealing load in Table 1 are obtained with .sigma.=200 MPa.
However, if the flange thickness, bolt number, etc. are set to their respective predetermined values in accordance with the recommendation standards, the following awkward situations sometimes may be entailed, although the satisfactory sealing performance can naturally be maintained.
A great number of ports of the plasma vacuum vessel are connected individually with the various apparatuses, so that the intervals between the ports are very short. Therefore, the spaces for the assembling and disassembling work for the flange assemblies are very narrow, so that the work is highly troublesome.
According to the aforementioned recommendation standards, moreover, the bolt number and the flange thickness are set to relatively great values. Thus, the assembling and disassembling work for the flange assemblies is highly troublesome, and the flanges are not very handy to carry.
According to the conventional standards, furthermore, the bolt tightening torque is so great that the maximum permissible tensile stress of the bolts is exceeded As a result, the bolts are subjected to plastic deformation, and in the worst case, may be damaged.
After the flanges are assembled with the ports, the vacuum vessel and the flange assemblies are baked (more specifically, heated and degassed) in order that impurities are removed from the vessel to form an ultrahigh vacuum therein At the same time, the flanges and the gaskets sometimes may be expanded and subjected to plastic deformation. If the vacuum vessel is cooled thereafter, the flanges and the gaskets are contracted in the deformed state. As a result, gaps are formed between the extremities of the knife edges and the gaskets, and air penetrates through the gaps into the vacuum vessel. In other words, a leakage is caused.
According to the conventional recommendation standards, as described above, the maximum permissible tensile stress of the bolts sometimes may be exceeded so that the bolts are subjected to plastic deformation. When the vacuum vessel is cooled after baking, in such a case, gaps are very likely to be formed between the knife edges and the gaskets In consequence, there is a fair chance for a leakage
Thus, although the conventional recommendation standards ensure the satisfactory sealing performance, they are subject to the various drawbacks.